Linking Coseismic Groundwater Elevation Changes to Stress and Pore Pressure Evolution through 2D Hydro-Mechanical Coupled Dynamic Distinct Element Modelling

Coseismic responses in groundwater level have often been observed following earthquakes worldwide. These responses have often been attributed to coseismic static and dynamic changes in volumetric strain and pore pressure, caused by slip on the ruptured fault. On 12. September 2016, the M_L 5.8 Gyeongju earthquake ruptured a branch of the Yangsan fault network in southeastern Korea, triggering hydrological responses near the mainshock epicenter. To better understand the connection between volumetric strain, pore pressure and groundwater level (GWT) levels, we developed a hydro-mechanical coupled dynamic distinct element model (dyn-DEM) to simulate the Gyeongju earthquake rupture process and subsequent fluid pressure response, using 2D Particle Flow Code v7. The rock mass was modeled using an assembly of circular particles, bonded to each other by contacts with the potential to break, collectively simulating the hydro-mechanical effects of a seismic event upon application of an in-situ stress field. The hydraulic fracture process was represented by a pipe network model, where fluid flow was simulated through a network of flow channels which connected pore spaces storing fluid volume and pressure. During the simulation, the finite volume method was used to solve for the pore pressure evolution due to poroelastic effect. Overall, we observed a positive correlation between coseismic GWT level changes near the Gyeongju earthquake epicenter and modeled stress and pore pressure changes. This result supports the use of hydro-mechanical coupled dyn-DEM in reliably quantifying changes in the stress and pore pressure fields throughout the dynamic rupture process of a simulated seismic event.